There is known a differential driver having a plurality of switches coupled to a current source for steering of current depending on a differential data input end (refer to Patent Document 1). A first differential output end and a second differential output end are formed by a resistor coupled between at least two of the plurality of switches. A first source follower and a second source follower are coupled to the first differential output end and the second differential output end in order to control output impedance.
Further, there is known a semiconductor integrated circuit having a current output buffer circuit which is driven by a constant current, and in which output impedance is controlled corresponding to a bit rate of differential transmission signal input (refer to Patent Document 2). A signal waveform to be outputted from the current output buffer circuit to a signal transmission path is controlled corresponding to the bit rate of the transmission signal.
Further, there is known an amplifier circuit having an amplifying part whose mutual conductance changes depending on a bias current (refer to Patent Document 3). A constant voltage source outputs a constant voltage. A constant current source outputs a constant current. A differential pair is composed of a pair of transistors having differential inputs to which the constant voltage is inputted, and the constant current is supplied through an output end of one of the pair of transistors. A pair of input current terminals is connected to the output ends of the pair of transistors. A difference current detection means outputs a voltage signal proportional to a difference output current of the differential pair. Each of first and second voltage-current conversion means receives the voltage signal as an input signal, and outputs current proportional to the voltage signal. The output currents by the first and second voltage-current conversion means compose bias currents of the differential pair and the amplifying part respectively.
[Patent Document 1] Japanese National Publication of International Patent Application No. 2009-531925
[Patent Document 2] Japanese Laid-open Patent Publication No. 2008-147940
[Patent Document 3] Japanese Laid-open Patent Publication No. 2001-251149
In a transmission circuit, trying to make amplitude of an output signal large results in small output impedance, and thus it becomes difficult to take impedance matching. It is difficult to maintain the output impedance at a predetermined value (for example, 50Ω) and at the same time make the amplitude of the output signal of the transmission circuit large in order to take the impedance matching.